1. Field of the Invention
The present invention relates to a decentration amount measuring method which measures the decentration amount of a surface to be measured of an optical element such as a lens, and particularly to a decentration amount measuring method which may specify the position of each indicator image formed on an image forming surface by reflected light from two-sided surfaces of a subject to be measured.
2. Description of the Related Art
As an important item for evaluation or check of a lens, there is measurement of the decentration amount of a lens surface.
As a method of measuring the decentration amount, for example, there has been known a method called autocollimation which uses a reflection-type decentration measuring apparatus shown in FIG. 13 (JP-A-2007-327771). This decentration measuring apparatus includes a mount 122 on which a lens to be measured (spherical lens) 110 is set rotatably with its center axis as a center, a light source 111, an indicator plate (pinhole plate, reticle plate) 112 which transmits the light from the light source 111, a measuring optical system 131 which irradiates the lens 110 with the light along the optical axis, and an image pick-up means for observing the light from the lens 110. Further, the measuring optical system 131 includes a beam splitter 113 which reflects the light which has been emitted from the light source 111 and has passed through the indicator plate 112 at the nearly right angle, a collimator lens 114 which makes the light from the beam splitter 113 into parallel beam, and an objective lens 115 which makes this parallel beam converge on a paraxial focus of the lens 110. Further, the image pick-up means is constituted by a CCD camera 121 having an image pick-up surface which observes the light from the lens 110 which has passes through the beam splitter 113.
When the amount of decentration is measured using the above decentration measuring apparatus, the lens 110 is irradiated with the light from the light source 111 which has passed through the indicator plate 112 by the measuring optical system 131.
At this time, by moving an optical converging position P of the objective lens 115, the optical converging position P is matched with the center of curvature of the surface to be measured (upper surface) of the lens 110. The light which has entered the surface to be measured of the lens 110, since it is regarded as equivalent to the light emitted from the center of curvature of this surface to be measured, is reflected from the surface to be measured so as to move reversely the incident path. This reflection light is made to enter the CCD camera 121 through the beam splitter 113. Thereafter, when the reflection image from the lens 110 is observed while the mount 122 is being rotated, in case that there is decentration, a locus of the pinhole images draws a circle. By calculating the radius of this circle, the decentration amount of the lens 110 may be found.
In the lens 110, a lower surface to be measured (setting surface) on the opposite side to the side of the upper surface to be measured is supported on the mount. Since this lower surface is also a spherical surface, even in case that the lens 110 is shifted on the mount 122, the position of the center of curvature of the lower surface does not change basically. In such the decentration measuring apparatus, the obtained measurement value of the decentration amount is taken as the final decentration amount of the surface to be measured as it is.
However, in case that the decentration amount of the lens 110 is found by the above method, two indicator images by the reflection light from the two-sided surfaces of the subject to be measured are formed on the image pick-up surface, and there is possibility that their images overlap with each other. Therefore, it is difficult to measure accurately the decentration amount of each surface on the basis of the indicator image.